I am a student at secondary school interested in physics and there is a problem I've been thinking about for two days: I just can't find out why the following perpetuum mobile doesn't work. Suppose we let two charges Q and -Q advance towards each other in air. We exploit the work done by the attractive force then push them into an oil tank where the attractive force is lower and using some of the energy gained previously we pull them apart until they reach their initial positions and begin the cycle again. I expect an answer of theoretical, not practical nature. Thx This problem really doesn't let me sleep.